The present invention relates to a directional display, for instance of the autostereoscopic three dimensional (3D) type. Such displays may be used in office environment equipment, laptop and personal computers, personal entertainment systems such as computer games, 3D television, medical imaging, virtual reality, videophones and arcade video games. The present invention also relates to a method of making a mask for a directional display.
FIG. 1(a) is a horizontal cross-sectional diagrammatic view of a known type of autostereoscopic 3D display, for instance as disclosed in EP 0 625 861, EP 0 726,482 and EP 0 721 131. The display comprises a diffuse or Lambertian backlight 1 disposed behind a spatial light modulator (SLM) 2 in the form of a liquid crystal device (LCD). The SLM 2 comprises a plurality of picture elements (pixels) such as 3 and the pixels are arranged in groups of columns. In the example illustrated, there are three columns in each group to provide a three window display. The columns are laterally contiguous as disclosed in EP 0 625 861 and as illustrated in FIG. 3, where the pixels 3 have apertures defined by an opaque black mask 11. The edge 12 of each column of pixels is contiguous with the edge of the adjacent column. A lenticular screen 4 is disposed in front of the SLM 2 with each lenticule being aligned with a corresponding group of three columns of pixels.
In use, the columns of each group display vertical slices of three different two dimensional (2D) images taken from different view points so that the 2D images are spatially multiplexed. Each lenticule such as 5 images light passing through the associated group of three pixel columns into wedge-shaped regions which form three viewing zones of a zeroth order lobe. Each lenticule 5 also images the groups of pixel columns aligned with an adjacent lenticule into repeated viewing zones of higher lobe order. The viewing zones are angularly contiguous.
In order to provide viewpoint correction so that each eye of the observer sees the same 2D view across the whole of the display, the pitch of the lenticules 5 of the lenticular screen 4 is slightly less than the pitch of the groups of pixel columns of the SLM 2. As illustrated in FIG. 2, the viewing zones thus define viewing windows 7 and 8 at the designed viewing distance of the display such that these windows lie in a plane parallel to the display and have the widest lateral extent at the window plane within the viewing zones. Provided the eyes 9 and 10 of the observer are located in adjacent viewing zones, for instance in adjacent windows 7 and 8, a 3D image is perceived without the need for the observer to wear viewing aids.
FIG. 1(b) shows another 3D autostereoscopic display which differs from that shown in FIG. 1 in that the lenticular screen 4 is replaced by a parallax barrier 6. Each of the lenticules 5 is thus replaced by a vertical slit which cooperates with the adjacent group of three pixel columns to define the viewing zones and the viewing windows of the zeroth order.
FIG. 1 (c) discloses a display which differs from that shown in FIG. 1 (b) in that the parallax barrier 6 is disposed between the SLM 2 and the backlight 1. The parallax barrier 6 is shown as being formed on a substrate of the SLM 2.
GB 9616281.3 and EP97305757.3 disclose an SLM which is particularly suitable for use in rear-illuminated autostereoscopic displays. Diffraction of light caused by transmission through pixels of the SLM causes degradation of the viewing zones. In order to reduce the diffraction spreading of the transmitted light, a complex transmission profile is imposed on the pixel apertures to modify the aperture profile and reduce the higher angular orders of diffractive spreading.
U.S. Pat. No. 4,717,949 discloses an autostereoscopic display which differs from that shown in FIG. 1(c) in that the backlight 1 and the parallax barrier 6 are replaced by an arrangement for forming a plurality of emissive light lines such as 13 as shown in FIG. 4. U.S. Pat. No. 5,457,574 discloses a specific arrangement for producing such lightlines as shown in FIG. 5. Light from a backlight 1 passes through a diffuser 14 and is collected by a Fresnel lens 15. The Fresnel lens 15 collimates the light from the backlight 1 and the diffuser 14 and supplies the collimated light to a lenticular screen 16. The lenticular screen 16 forms images of the diffuser 14 on a weak diffuser 17 so as to form the lightlines. Light from these lightlines is modulated by a spatial light modulator 2 and light efficiency is improved by another Fresnel lens 18 which restricts the illumination from the display to the region in space where an observer will be located.
Other known front lenticular screen and front parallax barrier autostereoscopic displays are disclosed in: G. R. Chamberlin, D. E. Sheat, D. J. McCartney, xe2x80x9cThree Dimensional Imaging for Video Telephonyxe2x80x9d, TAO First International Symposium, (Dec. 1993); M. R. Jewell, G. R. Chamberlain, D. E. Sheat, P. Cochrane, D. J. McCartney, xe2x80x9c3-D Imaging Systems for Video Communication Applicationsxe2x80x9d, SPIE Vol. 2409 pp 4-10 (1995); M. Sakata, C. Hamagishi, A. Yamasjita, K. Mashitani, E. Nakayama, xe2x80x9c3-D Displays without Special Glasses by Image-Splitter Methodxe2x80x9d, 3D Image Conference ""95; and JP 7-287198.
FIG. 6 illustrates the principle of operation of the rear parallax barrier display shown in FIG. 1 (c). The parallax barrier is a flat opaque screen with a series of thin transmitting slits 19 having a regular lateral pitch "sgr" and forming vertical illumination lines behind the LCD 2 when the backlight 1 is activated. The LCD 2 comprises pixel columns having a regular lateral pitch p. A number N of images are interlaced in adjacent vertical columns of pixels on the LCD and the parallax barrier pitch is approximately given by:
"sgr"=Npxe2x80x83xe2x80x83(1)
Therefore, for each column of pixels, there is a defined range of angles of illumination as shown at xcex8 due to the associated light line in the rear parallax barrier.
So that an observer""s eye located in the optimum viewing plane can only see one of the interlaced images displayed on the LCD 2, the pitch of the rear parallax barrier is designed to be slightly greater than that given by equation (1) so that the range of angles of view for each pixel column converge on the optimum viewing position. This is shown by the rays traced in FIG. 7 for a display showing two images. This pitch correction is known as xe2x80x9cviewpoint correctionxe2x80x9d and ensures that, at any given point on the viewing plane containing the viewing windows 7, 8, the parallax barrier slit 19 is visible at the same horizontal position within each pixel of one view. Moving laterally In the viewing plane causes the slit position to move within the pixels and ultimately be visible behind the adjacent columns of pixels. At this position the observer is in the next viewing zone. Hence the interlaced images and the parallax barrier 6 give rise to the viewing windows 7, 8, in the viewing plane within. which only one view is visible across the whole of the display. The viewpoint corrected pitch of a rear parallax barrier may be calculated from
"sgr"=Np(1+t/nL)
where t is the separation of the pixel plane and the barrier slits, n is the composite refractive index of the medium in this spacing and L is the optimum viewing distance of the display.
The front parallax barrier display operates in a substantially similar way. In this case the pixels are occluded by opaque parts of the mask outside of the viewing zones and visible through the slits in the viewing zones. FIG. 8 shows the viewing geometry of such a system.
Geometrical arguments lead to a first approximation of the viewing window intensity profile. If the LCD 2 has rectangular pixels, the viewing windows have uniform illumination across their central region. The illumination profile at the edges is sloped linearly due to the partial occlusion of the rear slit as the lateral position in the viewing plane causes the parallax barrier slit to move under the pixel edge. Therefore a wide rear slit gives a gentle slope to the edge of the viewing window and a narrow rear slit gives sharp edged viewing regions. A compromise between window edge function and light throughput decides the optimum parallax barrier slit width. FIG. 9 shows the ideal geometrical viewing window intensity profile as given by the dimensions shown in FIG. 7.
If the pixel apertures as defined by the black mask in the LCD are not rectangular, then the viewing window profile is not this simple trapezoidal or xe2x80x9ctentxe2x80x9d shape. At different lateral positions within the viewing plane, the observer will see the light lines through different vertical aperture sizes within the pixel. Thus the viewing window intensity follows the vertical extent of the pixel apertures blurred by the finite rear slit width. FIG. 10 shows an example of this. Gaps between adjacent columns of pixels will lead to dark areas in the viewing plane due to total occlusion of the parallax barrier light lines.
The geometrical performance of such displays is modified by diffraction effects. Fresnel diffraction effects are observed in the near-fields at apertures or obstacles in an optical path. This either means that the observer is very close to the aperture/obstacle or the light source is close behind the aperture/obstacle. Both cases are analogous in that they take into account the curvature of the wavefront. Fraunhofer diffraction is a simplified theory of the far-field case of diffraction and can be obtained by various simplifications in the Fresnel analysis including neglecting wavefront curvature effects and assuming a plane wave approach, Rear and front parallax barrier displays generally cause very different diffraction effects. The effect of diffraction in rear parallax barrier displays will be described in detail hereinafter.
To give a typical example, in the rear parallax barrier display of FIG. 1(c), the observer is looking at pixel apertures of 90 xcexcm width from 600 millimetres away. Therefore Fraunhofer diffraction would seem to be applicable for the light distribution caused by these. However, the action of the rear parallax barrier is to provide a defined source of light very close (1.3 mm) behind the pixel apertures.
Each point in the rear slit acts like a point source emitting spherical wavefronts due the diffuse rear illumination. As the slit is relatively narrow, these wavefronts do not combine to generate a plane wave across the pixel aperture width. Hence the illumination wavefront is not plane at the pixel aperture and Fresnel diffraction results. FIG. 11 shows this. If the rear slit was larger than the pixel aperture, then the wavefront across the pixel aperture would be essentially plane and Fraunhofer treatment would be appropriate.
The theory of Fresnel diffraction is covered in many appropriate textbooks, such as E. Hecht, xe2x80x9cOpticsxe2x80x9d, 2nd Ed. (Addison-Wesley, 1987).
The basic geometry is shown in FIG. 12 and consists of a point source at a distance xcfx810 behind an aperture of width w. A straight line connects the point to an observer P through the aperture (at 0) and defines an origin line. The observer is at a distance r0 from the aperture. The contributions to the amplitude received by the observer are summed over the aperture taking into account the phase of the curved wavefront within the aperture.
To calculate the intensity pattern received on the observation plane, the SOP line is considered fixed and the aperture is offset relative to this to give the effect of the observer""s motion. Thus the limits of integration for the aperture width are changed to follow the movement of the origin point within the aperture. Full details are given in the textbooks but the final result for an infinitely long slit is as follows. The intensity I(x) received at the lateral position x in the observation plane is given by:
I(x)=|B(u1(x),u2(x))|2
where u1 and u2 are the limits of integration for the Fresnel integrals and are defined as:
u1(x)=(x+w/2)[2(xcfx810+r0) /(xcexr0xcfx810)]xc2xd
u2(x)=(xxe2x88x92w/2)[2(xcfx810+r0) /(xcexr0xcfx810)]xc2xd
The Fresnel integrals themselves are given by:
B(u1(x),u2(x))=FR1(u2(x),u1 (x))+iFR2(u2(x), u1(x))
where             FR1      ⁡              (                  b          ,          a                )              =                  ∫        a        b            ⁢                        cos          ⁡                      (                          π              ⁢                              xe2x80x83                            ⁢                                                w                  2                                /                2                                      )                          ⁢                  ⅆ          w                                FR2      ⁡              (                  b          ,          a                )              =                  ∫        a        b            ⁢                        sin          ⁡                      (                          π              ⁢                              xe2x80x83                            ⁢                                                w                  2                                /                2                                      )                          ⁢                  ⅆ          w                    
An example diffraction pattern from a 90 xcexcm slit with a point source 1.3 xcexcm behind viewed from 600xcexcm is given in FIG. 13. These parameters are typical for a current display system. The back working distance (1.3 xcexcm) defines the wavefront curvature (xcfx810) of the incident light at the aperture. The pattern is complex with many sub-fringes.
The complete model for the viewing window intensity profile relies on the viewpoint correction between the rear parallax barrier 6 and the pixel layout as described hereinbefore. This pitch correction between the two components assures that, from the viewing plane, each pixel associated with a viewing window has a parallax barrier slit 19 behind it located in the same horizontal position relative to the pixel aperture 20. This horizontal offset changes as the observer moves laterally in the viewing plane as described above. The intensity received at a point in the viewing plane is the sum of the contributions from the Fresnel diffraction patterns produced by the pixels and, due to the viewpoint correction, every pixel gives the same contribution as the point source is located in the same position behind each pixel. Furthermore, as the observer moves laterally, the intensity pattern will follow the Fresnel diffraction pattern for a single slit. This is because movement in the viewing plane causes the slit to move behind every pixel by the same amount. This displacement of the source changes the diffraction effects which follow the diffraction profile calculated above. This is shown schematically in FIG. 14. Therefore, the viewing window intensity profile is merely a magnified version of the
Fresnel diffraction pattern from a single slit and can therefore be expected to be non-uniform if significant diffraction is occurring.
The assumption of slit apertures in the theoretical treatment (as opposed to rectangular apertures which are closer to the actual pixel shape) is valid as the pixels add up in columns to give a long vertical extent and any diffraction in the vertical plane is washed out by this. For a non-rectangular aperture a more complex treatment is appropriate which may be derived along the same lines as this simplified treatment. The finite width of the rear parallax barrier slits 19 needs to be taken into account however. This is done by considering the slit width to be an integration of point sources across itself and the final diffraction pattern is generated as the sum of the patterns from all the individual point sources.
Mathematically this integration is combined with the Fresnel diffraction integration by a convolution defined as follows. The Fresnel pattern I(x) is convolved with a top-hat function R(x) which mirrors the rear slit transmission function to give the viewing window profile V(x) in the usual manner:       V    ⁡          (      x      )        =            ∫              -        ∞            ∞        ⁢                  l        ⁡                  (          t          )                    ⁢              R        ⁡                  (                      t            -            x                    )                    ⁢              ⅆ        t            
The window profile produced after convolution with the rear slit width is shown in FIG. 15. This profile is generated from the data used in FIG. 13 and a rear slit width of 25 xcexcm, again a typical figure for a practical display.
Another complication is that the incident light is not monochromatic but is white. The theory assumes monochromatic light and a second convolution due to the range of wavelengths should strictly be performed. This is not accounted for in the present mathematics but would lead to a slight further blurring of the pattern.
According to a first aspect of the invention, there is provided a directional display comprising. a display arrangement for producing a plurality of viewing zones, each of which has a non-uniform first angular intensity profile with a first angularly varying component, characterised by a compensator for superimposing in the viewing zones a second angular intensity profile having a second angularly varying component which is substantially the inverse of the first angularly varying component.
The display arrangement may comprise a spatial light modulator having a plurality of picture elements and an array of discrete light sources. The picture elements may be arranged as columns and the light sources may comprise parallel evenly spaced line sources.
The light sources may comprise a diffuse backlight and a parallax barrier.
The parallax barrier may comprise a plurality of slits, each of which cooperates with a respective group of the picture element columns to form the viewing zones of a zeroth order lobe.
The picture elements may be of substantially constant vertical aperture, the spatial light modulator and the parallax barrier may cooperate to produce Fresnel diffraction and the compensator may be arranged to compensate for the non-uniform first angular intensity profile caused by the Fresnel diffraction.
The picture elements may be of non-constant vertical aperture, the spatial light modulator and the parallax barrier may cooperate to produce Fresnel diffraction, and the compensator may be arranged to compensate for the non-uniform first angular intensity profile caused by the non-constant vertical aperture and the Fresnel diffraction.
The compensator may comprise a mask disposed between the parallax barrier and the backlight and comprising a plurality of strips of varying light transmissivity which cooperate with the slits of the parallax barrier to form the second angularly varying intensity profile.
The strips may be of substantially the same width as the picture element columns.
The ratio of the lateral pitches of the strips and the slits may be substantially equal to the ratio of the lateral pitches of the slits and the groups of the picture element columns.
The parallax barrier and the mask may be formed on opposite faces of a common transparent substrate.
n1t1 may be equal to n2t2, where n1 is the effective refractive index between the spatial light modulator and the parallax barrier, t1 is the thickness between a picture element plane of the spatial light modulator and the parallax barrier, n2 is the effective refractive index between the parallax barrier and the mask, and t2 is the thickness between the parallax barrier and the mask.
A lenticular screen may be disposed between the mask and the parallax barrier. The lenticular screen may comprise a plurality of lenticules, each of which is aligned with a respective strip of the mask.
A switchable diffuser may be disposed between the spatial light modulator and the array of light sources and may be switchable between a diffusing mode and a substantially non-diffusing mode. The switchable diffuser may comprise a polymer dispersed liquid crystal layer.
The display arrangement may comprise a diffuse backlight, a parallax barrier, and a spatial light modulator disposed between the backlight and the parallax barrier.
The spatial light modulator may comprise a plurality of picture element columns and the parallax barrier may comprise a plurality of parallel evenly spaced slits, each of which cooperates with a respective group of the picture element columns to form the viewing zones of a zeroth order lobe.
The compensator may comprise a mask comprising a plurality of strips of varying light transmissivity, the parallax barrier may be disposed between the spatial light modulator and the mask, and each strip may cooperate with a respective slit to form the second angularly varying intensity pattern.
The compensator may comprise means for defining the aperture transmission properties of the picture elements.
The defining means may comprise a spatial light modulator black mask defining the shape of picture element apertures.
The defining means may spatially vary the transmissivity of picture element apertures.
The parallax barrier may comprise a first polariser, a second polariser and a polarisation modifying layer disposed between the first and second polarisers and having slit regions and barrier regions for supplying light of orthogonal polarisations.
The second polariser may comprise part of the spatial light modulator.
The first polariser may be removable to provide a non-directional mode of operation.
According to a second aspect of the invention, there is provided a method of making a mask for an embodiment of the display according to the first aspect of the invention, comprising disposing a photosensitive material in a plane substantially parallel to the spatial light modulator and intersected by the viewing zones, operating the display with the picture elements being transmissive so as to expose the photosensitive material, and reducing and repeating the image recorded by the photosensitive material.
The parallax barrier may be replaced by a further parallax barrier of reduced slit width during exposure of the photosensitive material.
The display may be viewpoint corrected to form viewing windows in a preferred viewing plane and the photosensitive material may be disposed at the viewing windows.
The mask may be formed on a transparent substrate of the parallax barrier.
According to a third aspect of the invention, there is provided a method of making a holographic mask for an embodiment of the display according to the first aspect of the invention of the viewpoint corrected type forming viewing windows in a preferred viewing plane, comprising disposing a photosensitive material at a parallax barrier plane with respect to the spatial light modulator, and exposing the photosensitive material by uniformly illuminating the viewing windows and supplying a front reference beam.
According to a fourth aspect of the invention, there is provided a method of making a holographic mask for an embodiment of the display according to the first aspect of the invention and of the viewpoint corrected type forming viewing windows in a preferred viewing plane, comprising disposing a photosensitive material in a parallax barrier position with respect to the viewing windows and exposing the photosensitive material by illuminating the viewing windows with a first intensity profile having a first spatially varying component which is substantially the inverse of a second spatially varying component of a second intensity profile produced by the display without the compensator and by supplying a front reference beam.
It is thus possible to provide a display which allows viewing zones and viewing windows to be generated with substantially improved uniformity of light intensity profile. For instance, variations caused by Fresnel diffraction can be substantially reduced. Further, for display arrangements having a pixel shape of non-constant vertical extent or aperture, the resulting illumination non-uniformity may also be substantially reduced. This allows SLMs having pixels of arbitrary shapes, such as existing SLMs, to be used, for instance in flat panel directional displays, while still producing a substantially uniform intensity of illumination within the viewing zones or viewing windows.
Such displays allow an observer to move laterally, for instance within the optimum viewing plane, without perceiving substantial variations in display brightness. For such displays which track movements of an observer, undesirable flicker artefacts are substantially reduced. Further, even if an observer is not located in the optimum-viewing plane, improvements in uniformity of display brightness over the whole display can be achieved. Also, freedom of viewing is extended laterally and longitudinally.
By compensating for diffraction effects within a display arrangement, the slit size of a rear parallax barrier may be reduced, This provides sharper edged viewing windows and provides greater viewing freedom.
It is also possible to provide a display which is switchable between 2D and 3D modes of operation, for instance using a switchable diffuser. Such a display has the advantages described hereinbefore in the 2D mode as well as in the 3D mode.